Fiber optic gyroscope asynchronous demodulation

ABSTRACT

A clock system for a fiber optic gyroscope is provided that includes a highly-tunable clock for the bias modulation and a separate asynchronous high-speed clock for the photodetector sampling. By separating the two clocks rather than using two derivatives of the same clock, the clock system and method can provide both the tunability objective of the bias modulation clock and the high-speed objective of the sampling clock, while using readily available, lower performance, radiation-hardened electronics parts.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under ContractN0030-04-C-0010 awarded by the Department of the Navy and under ContractF29601-03-0124 awarded by the Air Force Research Lab. The Government hascertain rights in this invention.

FIELD OF THE INVENTION

This invention generally relates to fiber optic gyroscopes, and morespecifically relates to timing in fiber optic gyroscopes.

BACKGROUND OF THE INVENTION

Fiber optic gyroscopes are used to accurately sense rotation of anobject supporting such a gyroscope. Fiber optic gyroscopes can be madequite small and can be constructed to withstand considerable mechanicalshock, temperature change, and other environmental extremes. Due to theabsence of moving parts, they can be nearly maintenance free.Furthermore, they can be highly sensitive to very low rotation ratesthat can be a problem in other kinds of optical gyroscopes.

A typical fiber optic gyroscope includes a coiled optical fiber wound ona core and about the axis around which rotation is to be sensed. Theoptical fiber provides a closed optical path in which an electromagneticwave is introduced and split into a pair of waves that propagate inopposite directions and ultimately impinge on a photodetector. Duringuse, a rotation about the sensing axis of the core provides an effectiveoptical path length increase in one rotational direction, and an opticalpath length decrease in the other rotational direction. The resultingpath length difference results in a phase shift between the wavespropagating in opposite directions. This result is generally referred toas the Sagnac effect. In a fiber optic gyroscope, the phase shiftresulting from the Sagnac effect is used to determine rotation aroundthe axis. Specifically, waves propagating in opposite directionsinterfere when recombined and impinge upon photodetector, which measurethe intensity of the combined wave. The output of the photodetector,which is a measure of the amount of interference, is used to determinethe phase difference in the counter-propagating beams, and thus is usedto determine rotation around the axis.

In many fiber optic gyroscopes, the traveling electromagnetic waves aremodulated by placing an optical phase modulator in the optical path onone or both sides of the coiled optical fiber. This modulation is usedto overcome directional ambiguity by introducing a phase shift to theincoming and outgoing waves in the optical fiber. As one example, thephase modulation is achieved by applying a modulating signal across theelectrodes of the optical phase modulator. Typically, the modulatingsignal is a square wave with a period equal to twice the transit time ofthe light through the coil. The modulating signal causes thephotodetector to measure the intensity at two different points in theraised cosine interferogram. The rotation rate and direction can then bedetermined by the difference in the emitted intensity at the twodifferent measured points.

In order to achieve a high level of performance, the bias modulationfrequency should equal the proper frequency of the fiber optic gyroscopesensing coil. The proper frequency is typically the frequency thatresults in the modulation of one of the counter-propagating waves 180degrees out of phase with the modulation of the other. The value of theproper frequency can be determined from the length of the optical fiberand the equivalent refractive index thereof. By modulating at the properfrequency, quadrature type errors are nearly eliminated. Quadratureerrors are unwanted signals that are synchronous with the desired ratesignal but are 90 degrees out of phase with the rate signal. The twomost common and significant quadrature errors in an fiber opticgyroscope are due to intensity modulation and second harmonic phasemodulation (or any even harmonics) generated by the bias phasemodulation. Both of these error mechanisms generate an optical signal atthe photodetector, which is 90 degrees out of phase with the desiredrotation rate signal. The quadrature error signals go to zero when thebias modulation frequency is adjusted to the proper frequency of thesensing coil. As a consequence it is highly desirable for a highperformance fiber optic gyroscope to generate the bias modulation at theproper frequency. Furthermore, it is desirable that the clock used togenerate the bias modulation frequency be tunable to account for coillength variance or to implement temperature-dependent proper frequencycompensation or a proper frequency servo.

In addition to generating a tunable clock to control the biasmodulation, a typical fiber optic gyroscope uses a high frequency clockto sample the photodetector signal. When sampling the photodetectorsignal, the samples at the beginning of each bias modulation period areusually rejected to eliminate the bias modulation glitch. A biasmodulation glitch is generated when the bias modulation frequency doesnot equal the proper frequency. For the difference in time between thechange in bias modulation and the loop transit time, bothcounter-propagating waves are equally phase shifted producing littleinterference when recombined. For this time period, a rate insensitivelarge photodetector signal or glitch is created with a widthproportional to the timing difference. Even if the bias modulationexactly equals the proper frequency, a bias modulation glitch can arisefrom the finite time it takes to modulate from one point of theinterferogram to another, caused by the slew limit of the biasmodulation drive signal. Anti-aliasing filters, with a frequencytypically set to half of the photodetector sampling frequency, are usedto prevent high frequency noise from aliasing to the demodulationfrequency, causing an increase in noise of the measured rotation rate.These same anti-aliasing filters also spread the bias modulationglitches creating the need to reject the photodetector samplessignificantly contaminated by the glitch. The larger the fraction of therejected samples, the larger the measured noise. By sampling thephotodetector signal at a higher frequency, the anti-aliasing filterfrequency can be set higher, decreasing the amount of glitch spreading.With less glitch spreading, fewer samples need to be rejected, resultingin improved noise performance. Therefore, for best noise performance, ahigher photodetector sampling frequency is preferred.

Turning now to FIG. 7, an embodiment of a high speed sampling clockgenerator Previous timing methods have provided a single tunablehigh-frequency clock from which both the bias modulation clock and thephotodetector sampling clock are derived. These methods have typicallyrelied upon high frequency tunable clock circuits that may not beavailable in all applications. For example, in applications whereradiation hardening is required (e.g. some space applications), thenumber and type of tunable high frequency circuits that are availablefor use is severely limited.

Thus, what is needed is a system and method for generating a highlytunable clock for the proper frequency and a high-speed clock for thephotodetector sampling converter that do not require unattainableradiation-hardened high speed devices.

BRIEF SUMMARY OF THE INVENTION

The present invention provides a clock system and method for a fiberoptic gyroscope that includes a highly-tunable clock for the biasmodulation and a separate asynchronous high-speed clock for thephotodetector sampling. By separating the two clocks rather than usingtwo derivatives of the same clock, the clock system and method canprovide both the tunability objective of the bias modulation clock andthe high-speed objective of the sampling clock, while using readilyavailable, lower performance, radiation-hardened electronics parts.

The preferred embodiment system for creating the tunable low-speed clockfor the bias modulation uses a direct digital synthesis (DDS) circuit. ADDS circuit is comprised of a digital sine wave generator, adigital-to-analog converter (DAC), an analog filter, and a comparator.Specifically, the digital sine wave generator creates a digital sinewave at a precisely controllable frequency. The digital sine wave isthen converted to an analog sine wave by the DAC. The DAC generated sinewave is then passed through a low pass filter. The filtered output, nowcomprised almost entirely of the desired frequency, is passed to acomparator. The output of the comparator is then a clock signal with ahighly tunable frequency and thus is suitable for use as a biasmodulation clock in a fiber optic gyroscope. Additionally, the DDS canbe implemented with relatively low frequency electronic components. Thisfacilitates the use of radiation hardened electronic components that aretypically only available at lower performance levels than non-radiationhardened components.

In addition to the bias modulation clock, the preferred embodimentseparately generates a relatively high-speed sampling clock. Thehigh-speed sampling clock is preferably generated by dividing down afixed-frequency high speed clock. For example, a fixed-frequency clockfrom a crystal oscillator can be divided down to the sampling frequencyof the analog-to-digital converter. Because the sampling clock isgenerated by dividing down the fixed-frequency high speed clock, it canbe provided using radiation hardened electronic components.

To measure rate, the photodetector signal is demodulated at the biasmodulation frequency by aligning the sampling clock with the transitionsof the bias modulation clock. Samples are then taken based on the sampleclock frequency until a predetermined number of samples have been taken.When the predetermined number of samples has been taken, no furthersamples are taken until the next appropriate transition of the biasmodulation clock.

The present invention thus provides a clock system and method that canachieve both the tunability objective of the bias modulation clock andthe high-speed objective of the sampling clock, while using readilyavailable, lower performance, radiation-hardened electronics parts.

BRIEF DESCRIPTION OF DRAWINGS

The preferred exemplary embodiment of the present invention willhereinafter be described in conjunction with the appended drawings,where like designations denote like elements, and:

FIG. 1 is a schematic view of an exemplary fiber optic gyroscope inaccordance with an embodiment of the invention;

FIGS. 2-5 are exemplary graphical views of cosine interferometerpatterns from counter-propagating waves

FIG. 6 is a schematic view of an exemplary bias modulation clockgenerator in accordance with an embodiment of the invention;

FIG. 7 is a schematic view of an exemplary sampling clock generator inaccordance with an embodiment of the invention; and

FIG. 8 is a graphical view of an exemplary timing diagram.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a clock system and method for a fiberoptic gyroscope that includes a highly-tunable clock for the biasmodulation and a separate asynchronous high-speed clock for thephotodetector sampling. By separating the two clocks rather than usingtwo derivatives of the same clock, the clock system and method canprovide both the tunability objective of the bias modulation clock andthe high-speed objective of the sampling clock, while using readilyavailable, lower performance, radiation-hardened electronics parts.

The preferred embodiment system for creating the tunable low-speed clockfor the bias modulation uses a direct digital synthesis (DDS) circuit. ADDS circuit is comprised of a digital sine wave generator, adigital-to-analog converter (DAC), an analog filter, and a comparator.The DDS circuit generates a clock signal with a highly tunable frequencyand thus is suitable for use as a bias modulation clock in a fiber opticgyroscope. In addition to the bias modulation clock, the preferredembodiment separately generates a relatively high-speed sampling clock.The high-speed sampling clock is preferably generated by dividing down afixed-frequency high speed clock. The present invention thus provides aclock system and method that can achieve both the tunability objectiveof the bias modulation clock and the high-speed objective of thesampling clock, while using readily available, lower performance,radiation-hardened electronics parts.

Fiber optic gyroscopes can sense rotation of an object supporting thefiber optic gyroscope. Such gyroscopes can be made quite small and canbe constructed to withstand considerable mechanical shock, temperaturechange, and other environmental extremes. Due to the absence ofsignificant moving parts they can be nearly maintenance free. They canalso be sensitive to low rotation rates that are difficult to detect inother kinds of gyroscopes.

Turning now to FIG. 1, an exemplary interferometric fiber opticgyroscope (IFOG) 100 is illustrated schematically. The IFOG 100 includesa light source 102, an optical coupler 104, an integrated optical chip106, a fiber optic coil 108, a detector 110, loop closure electronics112, and a bias modulator 114. Additionally, the IFOG 100 includes asampling clock generator 132 and a tunable bias modulation clockgenerator 130 that are part of a clock system that can achieve both thetunability objective for bias modulation and the high-speed objectivefor sampling, while using readily available, lower performance,radiation-hardened electronics parts It should be noted that the IFOG100 is merely exemplary of the type of IFOGs that the clock system canbe implemented in, and that other suitable implantations can includeother combinations of elements.

The fiber optic coil 108 is typically wound around a core and about anaxis around which rotation is sensed. The fiber optic coil 108 istypically long, on the order of between 50 and 2000 meters. The fiberoptic coil provides the closed optical path in which an electromagneticwave is split and propagates in opposite directions, ultimatelyimpinging on the detector 110. Rotation about the sensing axis in onedirection causes an effective increase in the optical path length forone direction, and a decrease in the optical path length in the otherdirection. The path length difference introduces a phase shift betweenthe waves, a result known as the Sagnac effect.

The coiling of optical fiber in the fiber optic coil 108 is desirablebecause the amount of phase shift due to rotation is dependent on thelength of the entire optical path through the coil traversed by the twowaves traveling in opposite directions. Therefore, a large phasedifference can be obtained in a long optical fiber that occupies arelatively small volume as a result of being coiled.

The light source 102 provides the light that propagates through the IFOG100. The light source 102 can be any suitable light source forpropagating electromagnetic waves through the fiber optics system 100.For example, the light source 102 can comprise pump laser that includesa semiconductor super luminescent diode. Alternatively, the light sourcecan comprise a rare earth doped fiber light source. Generally, it isdesirable that the light source provide a stable output of a selectedwavelength with relatively high efficiency.

The light source 102 is connected to the optical coupler 104 using asuitable optical path, typically comprising optical fibers. The opticalcoupler 104, sometimes referred to as a fiber coupler, optical lightbeam coupler or wave combiner and splitter, has light transmission mediawhich extend between four ports A, B, C and D. Port A is connected tothe light source 102, port B is connected to the detector 110, and portC is coupled to the integrated optical chip 106.

In general, when the optical coupler 104 receives electromagnetic wavesat any of its ports, the optical coupler 104 transmits the waves suchthat approximately half of the transmitted light appears at each of thetwo ports on the opposite end of the incoming port. At same time,substantially no electromagnetic waves are transmitted to the port whichis at the same end as the incoming port. For example, light received atport A will be transmitted to ports C and D, but will not besubstantially transmitted to port B. Similarly, waves received at port Cwill be transmitted to ports A and B, but not to port D, and so on.

Therefore, during operation light source 102 transmits light to port Aof the optical coupler 104. Optical coupler 104 splits the transmittedlight and provides the light to ports C and D. The light transmitted toport C is further transmitted to the integrated optical chip 106 viaoptical fiber or other suitable mechanism.

It should be noted that while the IFOG 100 illustrates the use of anoptical coupler 104, that other embodiments of the invention could usedifferent devices in place of the coupler 104. For example, in somefiber optic gyroscope implementations a circulator would be used insteadof an optical coupler.

The integrated optical chip 106 includes a Y-junction 120 and waveguides122 and 124. Additionally, integrated optical chip 106 includes anoptical phase modulator electrodes 126 integrated with the waveguides122 and 124. Again, it should be noted that while IFOG 100 uses anintegrated optical chip, that other embodiments of the invention coulduse other devices. For example, a separate Y-junction or coupler couldbe used along with a separate modulator.

Inside the integrated optical chip 106 the light is further split at theY-junction 120 and provided to waveguides 122 and 124. The lightprovided to wave guide 122 is transmitted to the fiber coil 108, whereit propagates clockwise around the length of the fiber coil 108. Thislight, referred to as the “cw wave”, returns to wave guide 124 in theintegrated optical chip 106 after propagating through the fiber coil108. Likewise, the light wave provided to wave guide 124 is transmittedto the fiber coil 108, where it propagates counter-clockwise around thelength of the fiber coil 108. This light, referred to as the “ccw wave”,returns to wave guide 122 in the integrated optical chip 106 afterpropagating though the fiber coil 108.

After being transmitted from the fiber coil 108 and passing throughtheir respective wave guides, the cw and ccw waves are combined at theY-junction 120 and propagate to port C of the optical coupler 104. Thecombined wave is then split and output at ports A and B. Port B isoptically coupled to the detector 110 such that the cw and ccw waves arereceived at the detector 110. The detector 110 typically comprises aphotodetector, such as an appropriate photodiode and a suitableamplifier. Of course, other suitable detectors can be used. When thecombined wave arrives at the detector 110, the detector 110 provides anoutput current proportional to the intensity of the two waves impingingon the photodetector 110.

The output of the detector 110 is passed to the loop closure electronics112. In general, the loop closure electronics 112 receives the currentfrom the detector 110 and drives the integrated optical chip 106 to afeedback phase shift needed to keep a difference in intensity for twomeasurements of the interferometer zero. Thus, the rotation ratemeasurement can be calculated from the feedback phase and delivered asoutput 116. It should be noted that while the IFOG 100 illustrates theof closed loop operation of the gyroscope, other embodiments of theinvention could use other implementations, such as open loop fiber opticgyroscopes that measure rotation directly from the demodulatedphotodetector signal.

When the two counter propagating waves impinge on the detector 110, theoutput from the detector 110 follows the cosine of the phase differencebetween the two waves. Turning now to FIG. 2, a graph 200 illustratesthe cosine of the phase difference between two exemplary waves impingingon the detector. The pattern illustrated in graph 200 is generallyreferred to as an interferometer pattern. Since the cosine is an evenfunction, such an output function does not give an indication as to therelative direction of the phase shift. Because of this, the phasedifference between the two counter-propagating waves are typicallymodulated, a process generally referred to as bias modulation.

The bias modulator 114 introduces a phase bias modulation by applying aphase shift to the incoming and outgoing waves in the integrated opticalchip 106. This modulation is achieved by applying a modulating voltageacross electrodes 126 around the wave guides 122 and 124. Theapplication of voltage lengthens or shortens the effective optical pathlength, thereby introducing an optical phase shift proportional to theapplied voltage. Typically, the bias modulator 114 applies a square witha half period equal to the transit time of the light through the coil.The bias modulation causes the detection points in the interferometer toshift, such that the detector 110 no longer measures the intensity atthe top of the interferometer, but rather at two offset points. Therotation rate, measured at the detector 110, is then proportional to thedifference in the emitted intensity at the two offset points. Turningnow to FIGS. 3 and 4, graphs 300 and 400 illustrate bias modulation onthe interferometer pattern. Specifically, graph 300 illustrates how asquare wave bias modulation signal 301 causes the detector to measurethe difference in emitted intensity at offset points 302 and 304. Thus,the rotation rate is now proportional to the difference in the emittedintensity at points 302 and 304. In graph 300, there is no rotation(Ω=0) and the difference in emitted intensity at points 302 and 304 iszero. In graph 400, there is rotation (Ω≠0) and thus the difference inemitted intensity between points 402 and 404 is non-zero.

In the preferred embodiment, the bias modulation signal φ_(M) isfrequency-tunable square wave with an amplitude of ±β/2·Vπ, where β isthe modulation depth and Vπ is the voltage required to induce a π radianphase shift in the integrated optical chip 106. The modulation depth, β,is typically selected to reduce random angle walk. The period of thebias modulation signal is tunable to half of the loop transit time ofthe light through the coil, such that the counter-propagating waveencounters the opposite modulation as the outgoing wave when it returnsto the integrated optical chip 106. When implemented such a modulationscheme phase advances the incoming wave and retards the outgoing wave,such that the two waves interfere when combined at the Y-junction 120.

As stated above, to achieve a high level of performance, the biasmodulation signal is made frequency tunable such that the period of themodulation signal can be made equal to twice the transit time of lightthrough the fiber optic coil 108. The frequency of the bias modulationsignal is controlled by the tunable bias modulation clock generator 130.The tunable bias modulation clock generator 130 provides a clock signalwith a frequency configured to cause the modulation of one of thecounter-propagating waves to be 180 degrees out of phase with themodulation of the other. The tunable bias modulation clock generator 130allows the frequency to be accurately adjusted to compensate forvariations in length of the optical fiber and the equivalent refractiveindex thereof. A detailed example of how tunable bias modulation clockgenerator 130 can generate the bias modulation clock will be discussedbelow.

As stated above, in closed loop operation, the loop closure electronics112 drives the integrated optical chip 106 to keep the difference inemitted intensity at the two offset measuring points zero. Thus, therotation rate measurement can be calculated from the feedback phase anddelivered as output 116. Turning now to FIG. 5, graph 500 illustrateshow a feedback phase shift φ_(FB), equal and opposite to therotation-induced phase shift, is introduced drive the difference inintensity at points 502 and 504 to zero. Thus, the rotation ratemeasurement can be calculated from the feedback phase shift φ_(FB).Specifically, the loop closure electronics can integrate the measuredfeedback phase shift φ_(FB) to obtain a value proportional to therotated angle. The rotated angle can then calculated by multiplying theaccumulated phase by a scaling factor.

Closed loop operation has many advantages over open loop operation. Forexample, by biasing to the same place on the raised cosineinterferometer the output is extraordinarily linear and stable.Additionally, because the output 116 is derived from a measurement ofphase, the output does not depend on the total intensity of measurementsat the detector, which could vary in response to changes in temperature,radiation, vibration and electronics gain.

For a constant rotation rate, the feedback modulation needed to closethe rate servo would be a continually increasing ramp. The loop closureelectronics 112 amplification circuitry is limited by its output range.To circumvent this problem, a voltage corresponding to a phase change of2π is added or subtracted to the feedback modulation, when the feedbackmodulation voltage nears the IOC 106 drive voltage limit.

As described above, the detector 110 receives the waves from the opticalcoupler 104 and provides an output current proportional to the intensityof the two waves. The output of the detector is passed to the loopclosure electronics 112. The loop closure electronics 112 samples theoutput of the detector 110 to determine the intensity of the two waves.The sampling of the detector 110 output is controlled by a samplingclock, which is created by the sampling clock generator 132. Thesampling clock is selected to have a frequency that will result in thedesired number of samples per half-period of the bias modulation clock.As will be described in greater detail below, the sampling clock ispreferably generated by dividing down a fixed-frequency high speedclock. For example, a fixed-frequency clock from a crystal oscillatorcan be divided down to the sampling frequency of the analog-to-digitalconverter. Because the sampling clock is generated by dividing downfixed-frequency high speed clock, it can be provided using radiationhardened electronic components.

The IFOG 100 thus includes a highly-tunable low-speed clock for biasmodulation and a separate non-tunable high-speed clock for thephotodetector sampling. By separating the two clocks rather than usingtwo derivatives of the same clock, one can achieve both the tunabilityobjective of the bias modulation clock and the high-speed objective ofthe photodetector sampling clock, while using readily available, lowerperformance, radiation-hardened electronics parts.

Turning now to FIG. 6, an embodiment of tunable bias modulation clockgenerator 600 is illustrated. This embodiment for creating the tunablelow-speed bias modulation clock is created using a direct digitalsynthesis (DDS) circuit. Specifically, the tunable low-speed biasmodulation clock comprises a digital sine wave generator 602, adigital-to-analog converter (DAC) 604, a filter 606, and a comparator608. The digital sine wave generator 602 creates a digital sine wave ata precisely controllable frequency. The frequency at which the digitalsine wave is generated would typically be adaptively controlled. Forexample, by accumulating an input phase step and using the rolloverfrequency of the accumulator as the desired frequency of the digitalsine wave. Of course, other suitable techniques can be used. The digitalsine wave is then converted to an analog signal by a DAC 602, which isclocked with a fixed frequency clock that is preferably at least fourtimes greater than the frequency of the sine wave created by the digitalsine wave generator 602. In one embodiment, the DAC clock is derivedfrom the same clock used to generate the photodetector sampling clock,as will be discussed in greater detail below. The DAC generated sinewave, with voltage quantization defined by the bit depth of the DAC andtime quantization defined by DAC clock rate, is then passed through thefilter 606. The filter 606 is preferably a low pass, analog filter. Alow pass filter preserves the desired sine wave frequency whilerejecting the frequencies generated by the DAC switching frequency. Inone embodiment, the filter 606 comprises a 5^(th) order Chebyshevfilter. The filtered output, now comprised almost entirely of thedesired frequency, is passed to the comparator 608 to generate a squarewave clock with a frequency that is independent of the clock used todrive the DAC 604.

In some embodiments it is desirable to generate the bias modulationclock at twice the necessary frequency so that only the rising edgesneed to be used to generate the bias modulation signal, removing anyrequirements on the duty cycle of the comparator output. Thus, a highlytunable clock can be generated, which is stable and frequency limited bythe speed of available components.

Additionally, because the bias modulation signal itself requires arelatively low frequency, the DDS-based modulator clock generator 600can be implemented with relatively low frequency electronic components.This facilitates the use of radiation hardened electronic componentsthat are typically available at much lower performance levels thannon-radiation hardened components. For example, for a fiber coil lengthof about 1 km, the proper frequency is near 100 kHz, and the desired DDSfrequency is double the proper frequency, ˜200 kHz. If the primary clockin the IFOG is a radiation-hardened 30 MHz fixed-frequency crystaloscillator, it can be divided by 20 with a counter to generate a 1.5 MHzclock for a radiation-hardened DAC. The 1 MHz DAC is faster than the DDSrequirement that the DAC frequency be at least four times faster thanthe proper frequency. The resultant ˜200 kHz clock is highly tunable andasynchronous to the 30 MHz fixed-frequency clock used to generate thesine wave. It should be noted that radiation-hardened DACs at such aspeed are available, and the entire circuit can thus be implemented withradiation-hardened electronics.

Turning now to FIG. 7, an embodiment of a high speed sampling clockgenerator 700 is illustrated. In this illustrated embodiment, thesampling clock generator 700 comprises a fixed high speed clock 702 anda frequency divider 704. The fixed high speed clock 702 can comprise anysuitable clock, such as a crystal oscillator that provides the mainfixed clock on the fiber optic gyroscope. The sampling clock is thusgenerated by dividing down a fixed-frequency high speed clock. A fixedfrequency clock can be used, since there are typically no requirementsfor tunability of the sampling clock. As one specific example, supposethe maximum sampling frequency of the analog to digital converter is 6MHz and the crystal oscillator frequency is 30 MHz, then the 6 MHzphotodetector sampling clock would be generated by using a counter todivide the 30 MHz clock by 5. The same high-speed crystal oscillatorclock can be used by the bias modulation clock generator to generate thebias modulation signal. For example, the same fixed frequency clock canbe divided down to the maximum conversion frequency of thedigital-to-analog converter used to generate the bias modulation clock,and can be divided down to the maximum sampling frequency of theanalog-to-digital converter used to sample the photodetector signal.

To best demodulate the photodetector signal, the photodetector samplesare preferably aligned with the edges of the bias modulation signal. Inthis clock system the bias modulation clock edges are asynchronous tothe photodetector sampling clock edges. To facilitate alignment, thebias modulation clock is sampled with the high-speed fixed frequencyoscillator clock to locate clock transitions in the bias modulationclock. In the embodiment where the bias modulation clock is generated attwice the bias modulation frequency, only a rising edge transition needsto be identified, and the method is immune to any duty cyclerequirements on the bias modulation clock.

Thus, during operation the bias modulation clock is sampled with thehigh speed fixed clock. When a clock transition is detected in the biasmodulation clock, the sampling clock is started and first photodetectorsample is taken within the coil loop transit time. Typically, thesampling clock can be started within a few high speed clock cycles.Thus, although the bias modulation clock and sampling clock areasynchronous, this facilitates sufficient alignment of the samplingclock with the bias modulation clock. Subsequent photodetector samplesare then taken based on the derived fixed frequency clock. When apredetermined number of photodetector samples have been taken, nofurther measurements are made until the next transition on the DDSgenerated clock.

Turning now to FIG. 8, an illustrative example of the clocking systemoperation is illustrated in graph 800. Graph 800 illustrates anexemplary high-speed fixed-frequency crystal oscillator clock at 30 MHz,the 1.5 MHz clock used to generate the digital sine wave in the DDS biasmodulation clock circuit, the resultant tunable bias modulationgenerated clock at 200 kHz, and the photodetector sampling clock at 6MHz closely aligned to the rising edge of the bias modulation clock. Inthis example there are 30 photodetector samples per bias modulationtransition, and no more samples are taken until the next bias modulationtransition.

One consequence of asynchronously clocking the bias modulation and thephotodetector sampling is a timing jitter of the photodetector samplesrelative to the bias modulation transition edge. This timing jitter isdetermined by the frequency of the clock used to recognize thetransition of the bias modulation clock. In the provided embodiment,this timing jitter is 30 MHz. Anti-aliasing filters are required on thephotodetector signal to prevent high frequency noise from aliasing tothe demodulation frequency. The frequency of the anti-aliasing filtersare typically set to half of the photodetector sampling frequency. Inthe provided embodiment, the photodetector sampling frequency is 6 MHzand the anti-aliasing filters are set to 3 MHz. As a consequence of theanti-aliasing filters at 3 MHz, the error incurred by the 30 MHz timingjitter is minimal.

The present invention thus provides a clock system and method for afiber optic gyroscope that includes a highly-tunable clock for the biasmodulation and a separate high-speed clock for the photodetectorsampling. By separating the two clocks rather than using two derivativesof the same clock, the clock system and method can provide both thetunability objective of the bias modulation clock and the high-speedobjective of the sampling clock, while using readily available, lowerperformance, radiation-hardened electronics parts.

The embodiments and examples set forth herein were presented in order tobest explain the present invention and its particular application and tothereby enable those skilled in the art to make and use the invention.However, those skilled in the art will recognize that the foregoingdescription and examples have been presented for the purposes ofillustration and example only. The description as set forth is notintended to be exhaustive or to limit the invention to the precise formdisclosed. Many modifications and variations are possible in light ofthe above teaching without departing from the spirit of the forthcomingclaims.

1. A clock system for a fiber optic gyroscope, the fiber optic gyroscopeincluding an optical path having a transit time, the clock systemcomprising: a bias modulation clock generator, the bias modulation clockgenerator generating a frequency tunable bias modulation clock signal;and a sampling clock generator, the sampling clock generator generatinga sampling clock signal asynchronous with respect to the bias modulationclock signal, wherein the sampling clock generator generates a firstsampling clock cycle in a bias modulation clock signal period inresponse to a transition in the bias modulation clock signal.
 2. Theclock system of claim 1 wherein the first sampling clock cycle isgenerated within the transit time of the transition in the biasmodulation clock signal.
 3. The clock system of claim 1 wherein thesampling clock generator further generates a predetermined additionalnumber of sampling clock cycles within the bias clock signal periodafter the first sampling clock cycle.
 4. The clock system of claim 1wherein the sampling clock generator further samples the bias modulationclock signal is with a relatively high frequency clock signal to locatethe transitions in the bias modulation clock signal.
 5. The clock systemof claim 1 wherein the sampling clock generator comprises a fixed clockand frequency divider, the fixed clock outputting a relatively highfrequency clock signal and the frequency divider dividing the relativelyhigh frequency clock to generate the sampling clock signal separate fromthe bias modulation clock signal.
 6. The clock system of claim 1 whereinthe bias modulation clock generator comprises direct digital synthesiscircuit.
 7. The clock system of claim 1 wherein the direct digitalsynthesis circuit comprises a digital sine wave generator, the digitalsine wave generator creating a digital sine wave approximation having acontrollable period, a digital-to-analog converter, thedigital-to-analog converter receiving the digital sine waveapproximation and outputting an analog sine wave signal, an analogfilter to filter the analog sine wave signal, and a comparator, thecomparator receiving the filtered analog sine wave signal and outputtingthe bias modulation clock signal.
 8. The clock system of claim 7 whereinthe digital sine wave generator, digital-to-analog converter, analogfilter, comparator, fixed clock and frequency divider each compriseradiation-hardened components.
 9. The clock system of claim 7 whereinthe digital-to-analog converter further receives a digital-to-analogconverter clock, and wherein the digital-to-analog converter clock isgenerated by dividing down a relatively high frequency clock signal. 10.A clock system for a fiber optic gyroscope, the fiber optic gyroscopeincluding an optical path having a transit time, the clock systemcomprising: a bias modulation clock generator, the bias modulation clockgenerator generating a frequency tunable bias modulation clock signal;and a sampling clock generator, the sampling clock generator generatinga sampling clock signal asynchronous with respect to the bias modulationclock signal, wherein the sampling clock generator generates a firstsampling clock cycle after a transition in the bias modulation clocksignal, and wherein the sampling clock generator further generates apredetennined additional number of sampling clock cycles within the biasclock signal period after the first sampling clock cycle.
 11. The clocksystem of claim 10 wherein the sampling clock generator samples the biasmodulation clock signal with a relatively high frequency clock signal tolocate the transitions in the bias modulation clock signal.
 12. Theclock system of claim 10 wherein the sampling clock generator comprisesa fixed clock and frequency divider, the fixed clock outputting arelatively high frequency clock signal and the frequency dividerdividing the relatively high frequency clock to generate the samplingclock signal separate from the bias modulation clock signal.
 13. Theclock system of claim 10 wherein the bias modulation clock generatorcomprises direct digital synthesis circuit.
 14. The clock system ofclaim 13 wherein the direct digital synthesis circuit comprises adigital sine wave generator, the digital sine wave generator creating adigital sine wave approximation having a controllable period, adigital-to-analog converter, the digital-to-analog converter receivingthe digital sine wave approximation and outputting an analog sine wavesignal, and a comparator, the comparator receiving the analog sine wavesignal and outputting the bias modulation clock signal.
 15. The clocksystem of claim 14 wherein the digital sine wave generator,digital-to-analog converter, comparator, fixed clock and frequencydivider each comprise radiation-hardened components.
 16. The clocksystem of claim 15 wherein the digital-to-analog converter furtherreceives a digital-to-analog converter clock, and wherein thedigital-to-analog converter clock is generated by dividing down arelatively high frequency clock signal.
 17. A clock system for a fiberoptic gyroscope, the fiber optic gyroscope including an optical pathhaving a transit time, the clock system comprising: a bias modulationclock generator, the bias modulation clock generator comprising: adigital sine wave generator, the digital sine wave generator creating adigital sine wave approximation having a controllable period; adigital-to-analog converter, the digital-to-analog converter receivingthe digital sine wave approximation and outputting an analog sine wavesignal; a comparator, the comparator receiving the analog sine wavesignal and outputting a bias modulation clock signal; and a samplingclock generator, the sampling clock generator comprising a fixed clockand frequency divider, the fixed clock outputting a relatively highfrequency clock signal and the frequency divider dividing the relativelyhigh free clock to generate a sampling clock signal asynchronous withrespect to the bias modulation clock signal, wherein the sampling clockgenerator generates a first sampling clock cycle in a bias modulationclock signal period in response to a transition in the bias modulationclock signal, and wherein the first sampling clock cycle is generatedwithin the transit time of the transition in the bias modulation clocksignal, and wherein the sampling clock generator further generates apredetermined additional number of sampling clock cycles within the biasclock signal period after the first sampling clock cycle.
 18. The clocksystem of claim 17 wherein the sampling clock generator samples the biasmodulation clock signal with the relatively high frequency clock signalto locate the transitions in the bias modulation clock signal.
 19. Theclock system of claim 17 wherein the digital sine wave generator,digital-to-analog converter, and comparator are implemented in a directdigital synthesis circuit and wherein the digital sine wave generator,digital-to-analog converter, comparator, fixed clock and frequencydivider each comprise radiation-hardened components.